As requirements for gas turbine emissions have become more stringent, one approach to meeting such requirements is to move from diffusion flame combustors to combustors utilizing lean fuel and air mixtures using a fully premixed operations mode to reduce emissions of, for example, NOx and CO. These combustors are generally known in the art as Dry Low NOx (DLN), Dry Low Emissions (DLE) or Lean Pre Mixed (LPM) combustion systems.
Certain DLN type combustors include a plurality of primary fuel nozzles which are annularly arranged about a secondary or center fuel nozzle. The fuel nozzles are circumferentially surrounded by an annular combustion liner. The combustion liner defines an upstream combustion chamber and a downstream combustion chamber of the combustor. The upstream combustion chamber and the downstream combustion chamber may be separated by a throat portion of the combustion liner.
During operation of the combustor, the primary fuel nozzles may provide fuel to the upstream combustion chamber. Depending on the operational mode, the fuel from the primary fuel nozzles may be burned in the upstream combustion chamber or may be premixed with compressed air within the upstream combustion chamber for ignition in the downstream combustion chamber. The secondary fuel nozzle serves several functions in the combustor including supplying fuel and air mixture to the downstream combustion chamber for premixed mode operation, supplying fuel and air for a pilot flame supporting primary nozzle operation and providing transfer fuel for utilization during changes between operation modes.
In certain combustors, the secondary fuel nozzle may include a diffusion pilot nozzle disposed at a downstream end of the secondary fuel nozzle. The diffusion pilot nozzle provides a stream of fuel and air to the second combustion chamber and is employed for anchoring a secondary flame. However, in order to comply with various emissions requirements the fuel flow to the pilot fuel circuit may be reduced. As a result, the reduced fuel flow to the pilot fuel circuit may impact combustion dynamics and/or lean blow out limits.